In wireless transmission links there often exists asymmetries. This is particularly true for cellular telecommunication networks. For example, the downlink can typically support a much higher bit-rate than the corresponding uplink. There are many reasons for such an asymmetry, but one fundamental reason is the fact that the downlink transmitter unit, the base-station, typically can use much higher transmission power compared to the corresponding uplink transmitter unit, the mobile terminal.
Other reasons include e.g. multi-user scheduling and link-adaptation issues: The down link transmitter is typically much better equipped to maintain orthogonality between transmissions to different users e.g. by time-multiplexing. However, multi-user scheduling and link-adaptation in the uplink can be very challenging as the transmitters are geographically distributed. It is also much simpler to introduce transmission diversity in the Multiple Input Multiple Output (MIMO) downlink because, compared to the uplink, the spatial spacing of multiple transmission antennas are not restricted by the terminal size. In general, it is therefore to expect higher bit-rates in the downlink direction compared to the uplink.
An example is the High-Speed Downlink Shared Channel (HS-DSCH) of the standard 3GPP Release-5, which can theoretically support bit-rates beyond 10 Mbps. In practice a couple of Mbps has been achieved in the first product releases. However, the reverse link, the uplink, of Release 5 is carried with dedicated channel DCH, typically supporting 64 kbps and up to 384 kbps in case of good coverage. Release 6 of the wideband code division multiple access WCDMA standard comes with the enhanced dedicated channel (E-DCH) enhancement of the up link. However, the supported bit-rates will still be far below the HS-DSCH downlink bit-rates, particularly at times when the network is loaded with multiple users.
A link of a cellular radio system network having different properties for the uplink and down link, respectively can be termed an “Asymmetric Link”. Moreover the bit rate ratio between the uplink and down link provided by the link-layer can be denoted “Asymmetry Ratio”. For example, if the downlink layer 2 carries 3.84 Mbps, but the corresponding up-link only 384 kbps, the Asymmetry Ratio is determined to be 10.
It is known that simultaneous upload and download with Transmission Control Protocol (TCP) over such an asymmetric link will constrain the throughput of the direction with the higher bit-rate to a level proportional to the narrower direction, see for example L. Kalampoukas, A. Varma, K. Ramakrishnan “Improving TCP throughput over two-way asymmetric links: Analysis and solutions”, Sigmetrics, 1998 ACM SIGMETRICS, http://citeseer.ist.psu.edu/cache/papers/cs/686/ftp:zSzzSzftp.cse.ucsc.eduzSzpubzSzhsnlabz SzSigmetrics98.pdf/kalampoukas98improving.pdf. The reason is that the load offered by TCP in forward direction is based on feedback that must be carried in the reverse direction. If the uplink is loaded and in particular over loaded by other traffic the result is that this feedback in the form of acknowledgement messages (ACKs) are delayed, which will prevent TCP from clocking out new data on the downlink path with the desired pace. This is described below in conjunction with FIG. 1 and FIG. 2.
Thus, in FIG. 1, the TCP sender in the Server maintains a transmission window, and each acknowledgement, illustrated by smaller packets 103 on the reverse link clocks out new data, large packets 101, from the Server. The downlink TCP connection can offer sufficient load to the downlink path only in case the acknowledgements on the reverse link arrive with sufficient pace.
FIG. 2 illustrates two simultaneous TCP connections, one in downlink illustrated by large packets 101 and small ACK packets 103 and one in uplink illustrated by large striped packets 201 and small striped ACK packets 203. Compared to the scenario illustrated in FIG. 1, the uplink will now be saturated with TCP segments 201 from the upload connection that will hinder the TCP ACKs 103 of the downlink connection from being transmitted at sufficient pace. This means that also the downlink performance is constrained by the uplink bit-rate.
Because link asymmetry is common in wireless radio system accesses and it will so remain, it is desired to find a solution that would enhance the down-link performance at times of simultaneous upload and download.